The activation of the efferent vestibular system and its action upon information transfer via the primary afferents of the vestibular system will be studied during natural vestibular stimulation. We will use glass microelectrodes and extracellular recording in toadfish, Opsanus tau. The long-term goal of these studies is to understand the purpose, mode of action and role of this efferent system. We will address the action of the system upon the response dynamics of the horizontal semicircular canal afferents to angular acceleration and will perform a structure-function study of the distribution of efferents and afferents within the crista of the canal. We will assess the determinants of activation of the efferent system in tethered and free swimming fish. We will determine the central distribution of the thick and thin fiber components of the horizontal semicircular canal nerve as it pertains to the selective activation of components of this nerve by the efferent system. An isolated, in-vitro canal preparation will be utilized to determine the synaptic events in hair cells following stimulation of the efferent vestibular nerve. This experiment will be crucial to our understanding of efferent function because of the differential effects of efferent activation upon components of the canal nerve. Immunohistological procedures will be used to localize putative peptide neurotransmitters or modulators within the central and peripheral efferent vestibular system.